Creating Captcha Code with PHP

A guide on how to create a simple Captcha using PHP to protect your website from spam and automated bots.

This article will show you how to use PHP to generate a random Captcha image, which helps improve website security by preventing spam from automated bots.

PHP Code

<?php
// Start session
session_start();

// Create a random Captcha string
$captcha_code = '';
$characters = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789';
$length = 6;

for ($i = 0; $i < $length; $i++) {
    $captcha_code .= $characters[rand(0, strlen($characters) - 1)];
}

// Store Captcha in session
$_SESSION['captcha'] = $captcha_code;

// Create Captcha image
$width = 150;
$height = 40;
$image = imagecreate($width, $height);

// Colors
$background_color = imagecolorallocate($image, 255, 255, 255); // White
$text_color = imagecolorallocate($image, 0, 0, 0); // Black

// Fill background
imagefilledrectangle($image, 0, 0, $width, $height, $background_color);

// Add Captcha string to image
$font_size = 20;
$angle = 0;
$x = 10;
$y = 30;
imagestring($image, $font_size, $x, $y, $captcha_code, $text_color);

// Output image to browser
header('Content-Type: image/png');
imagepng($image);

// Free up memory
imagedestroy($image);
?>

Detailed explanation:

1. session_start(): Starts a session to store the Captcha code.
2. The block of code generates a random Captcha string.
3. $_SESSION['captcha']: Stores the Captcha string in the session.
4. imagecreate(), imagecolorallocate(), imagefilledrectangle(), imagestring(): Functions to create the Captcha image with a white background and black text.
5. header('Content-Type: image/png'): Sets the HTTP header to output a PNG image.
6. imagepng($image): Outputs the image.
7. imagedestroy($image): Frees up memory.

System Requirements:

• PHP 7.0 or higher
• PHP GD extension (enabled by default)

How to install the libraries needed to run the PHP code above:

Ensure you have PHP installed with GD support. You can check using the php -m command in the terminal.

Tips:

• To increase complexity, consider adding curves, noise, or varying text colors and background.
• Always store the Captcha string in the session for verification on the validation page.

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